1. Field of the Invention
The present invention relates to a fixing apparatus for fixing a toner image.
2. Description of the Related Art
The conventional image forming apparatus such as printer and copier performs fixation of a toner image on a sheet by forming a toner image using an electrophotographic recording method, and by transferring the toner image onto the sheet, then by applying heat and pressure to the sheet to fix the toner image on the sheet.
That type of fixing apparatus for fixing the toner image onto the sheet includes the one, which has a metal roller having a heater therein, and has an elastic fixing belt in pressure contact with the metal roller.
FIG. 10 illustrates a general structure of a color laser printer as an example of an image forming apparatus having the conventional fixing apparatus of the above structure. In FIG. 10, a color laser printer 100 has a color laser printer main body (hereinafter referred to as the “printer main body”) 101, an image forming portion 102 to form an image on a sheet P, and a fixing apparatus 105.
In the color laser printer 100, after a toner image is formed by the image forming portion 102, the toner image is transferred onto the sheet P. Then, the sheet P is sent to the fixing apparatus 105. After that, the sheet P is heated and pressed in the fixing apparatus 105 to fix the toner image onto the sheet.
As illustrated in FIG. 11, the fixing apparatus 105 has a rotatable fixing roller 510 and a fixing belt unit 53. The fixing roller 510 is driven by a drive source (not shown) to rotate in a direction indicated by the arrow A, while being heated by a halogen heater 520 disposed inside fixing roller 510. The surface temperature of the fixing roller 510 is controlled by a thermistor 525 located on the surface of the fixing roller 510 to keep the temperature constant.
The fixing belt unit 53 has a fixing belt 531 which is an endless belt, and a pressure pad portion 540 which is brought into pressure contact with the fixing roller 510 through the fixing belt 531 in a direction indicated by the arrow PF to form a fixing nip.
Here, the fixing belt 531 is passed around an inlet roller 532, a separation roller 533, and a steering roller 534. The separation roller 533 is made of metal such as stainless steel, and is brought into pressure contact with the fixing roller 510 through the fixing belt 531 in a direction indicated by the arrow SF. One end of the steering roller 534 is movable in directions indicated by the double-headed arrow B. The deviation of the fixing belt 531 can be corrected by moving the one end of the steering roller 534.
The pressure pad portion 540 is disposed between the inlet roller 532 and the separation roller 533, and has a base 541 made of metal such as stainless steel, a pressure pad 542, and a slide sheet 543 disposed between the pressure pad 542 and the fixing belt 531. The slide sheet 543 is made of PI film, while the pressure pad 542 is made of silicon rubber.
An oil felt 536 impregnated with silicone oil is disposed between the inlet roller 532 and the pressure pad 540. The oil felt 536 applies oil onto an inner surface of the fixing belt 531 so that the friction force between the fixing belt 531 and the slide sheet 543 decreases.
Here, for that type of conventional fixing apparatus 105, unless the fixing roller 510 and the fixing belt 531 in a waiting operation are kept at a specified temperature, the next image formation output takes a time until the fixing roller 510 and the fixing belt 531 reach the specified temperature. Therefore, even in the waiting operation, the fixing roller 510 and the fixing belt 531 are required to be kept heated.
By the way, the uses of forming color image have been increasing in recent years. Accordingly, in addition to the ability of outputting an image of the normal quality, the image forming apparatus is required further to have the ability of outputting a high gloss image of a quality comparable to a silver halide photography. To this point, a coat paper is used in some cases as a sheet to improve the color-forming property of the toner image and the quality of the image. That type of coat paper is prepared by coating a coat layer comprising synthetic resin on the surface of the sheet at a thickness of several tens of micrometers.
When the fixing apparatus 105 fixes a toner image on a coat paper after the toner image is transferred onto the coat paper, if an excess heat is applied to the coat paper, the water in the coat paper evaporates and the thus evaporated water may partially destroys the coat layer applied on the surface of the sheet. If the coat layer is destroyed, the image loses the smoothness. In particular, the fixing apparatus using the fixing belt has a wide nip, which applies the large amount of heat to the coat paper, so that such a fixing apparatus likely poses the above problem.
In this regard, according to the related art, when the apparatus is in a waiting operation, contrary to the image forming operation mode illustrated in FIG. 12A, the fixing belt unit 53 is retreated from the fixing roller 510 as illustrated in FIG. 12B, thus assuring non-contact between the fixing belt 531 and the fixing roller 510. Even when the fixing belt 531 is separated from the fixing roller 510, a heater 535 is provided in the inlet roller 532 to keep the temperature of the fixing belt 531 at the specified temperature.
In the state illustrated in FIG. 12B, the drive source (not shown) rotates the fixing belt 531 in a direction indicated by the arrow C, and the heater 535 in the inlet roller 532 heats the fixing belt 531 through the inlet roller 532 to keep the temperature of the fixing belt 531 constant by a thermistor 537.
The temperature of the fixing belt 531 is set to a temperature lower than the temperature of the fixing roller 510 to some extent, thereby reducing the heat applied to the backside of the coat paper to suppress the evaporation of water in the coat paper, thus preventing the occurrence of the above phenomenon.
Referring back to FIG. 11, a sheet presence or absence detecting sensor 580 is located on the upstream side of the fixing apparatus 105 in the sheet conveying direction to detect the presence or absence of the sheet at the fixing nip. The sheet presence or absence detecting sensor 580 has a shaft 582, a sensor lever 583 oscillatable centering on the shaft 582, and a photo-interrupter 584.
The sensor lever 583 has a contact portion 583b which projects by a spring (not shown) from an opening (not shown) above a sheet guide 570 when there is no sheet on a sheet guide 570, and a shutter portion 583a to shield the photo-interrupter 584.
When there is no sheet in the fixing nip, the sensor lever 583 moves so as the contact portion 583b to project above the sheet guide 570, as illustrated in FIG. 11, and moves to a position where the shutter portion 583a does not shield the photo-interrupter 584. When a sheet is in the fixing nip, or when the sheet is passing through the fixing nip, the sensor lever 583 is pressed by the sheet P to oscillate about the shaft 582, thereby moving the shutter portion 583a to a position to shield the photo-interrupter 584.
Even when the shutter portion 583a shields the photo-interrupter 584, the controller (not shown) neglects the signal of the sheet presence or absence detecting sensor 580 as far as the conveying of the sheet P is correctly conducted.
Once jamming of sheet occurred in the printer main body, the controller (not shown) forcefully retreats the fixing belt unit 53 with respect to the fixing roller 510, as illustrated in FIG. 12B. Furthermore, the controller detects the presence or absence of the sheet based on the signal generated from the sheet presence or absence detection sensor 580, and when the controller determines that the jammed sheet exists in the fixing nip based on the signal generated from the sheet presence or absence detecting sensor 580, the controller indicates the presence of sheet in the fixing nip.
If, however, the sensor lever 583 is used to detect the presence or absence of jammed sheet at the fixing nip as described above, the pressing of the sensor lever 583 induces deformation of the leading edge of the sheet P, which worsens the shape of sheet entering the fixing nip, or which results in unstable behavior of sheet. Once the sheet entering shape becomes worse, or the sheet behavior becomes unstable, there is a possibility to cause the poor fixation (defective image) on fixing the toner image in the fixing nip.
A measure to prevent the leading edge deformation of sheet P is that the sheet presence or absence detecting sensor is formed as a photo-coupler integrating an infrared luminous element with an infrared light-receiving element to detect the presence or absence of the sheet by irradiating the infrared light to the backside of the sheet, (refer to, for example, Japanese Patent Application Laid-Open No. H06-175524).
With the conventional fixing apparatus and image forming apparatus detecting the presence or absence of a sheet using that type of photo-coupler, there is a need of forming a hole in the sheet guide to irradiate the infrared light to the backside of the sheet. With that hole, however, toner and paper dust may pass through the hole to adhere to the photo-coupler, which raises a problem of failing in the detection of the presence or absence of the sheet.
To this point, for example, it is considered to project the sensor lever of the sheet presence or absence detecting sensor to above the sheet guide only when the detection of the presence or absence of a sheet is required, while a solenoid usually retreats the sensor lever from the sheet guide(refer to, Japanese Patent Application Laid-Open No. H06-175524).
Although the above construction can prevent the deformation of leading edge of the sheet P, the construction presents other problems of a cost increase caused by the addition of a drive source such as a solenoid, an additional space for installing the solenoid, and an additional drive controller for the solenoid.
Furthermore, when the sensor lever is used to detect the presence or absence of the sheet, it is necessary to provide the sheet guide with an opening through which the sensor lever can be projected to above and retreated from the sheet guide. When, however, the sensor lever is retreated from the sheet guide, the leading edge of a sheet may be caught by the opening. Once the leading edge of the sheet is caught by the opening, the shape of the sheet entering fixing nip may be deteriorated so that there is a possibility that a poor fixing occurs.